This invention relates to a noise reduction circuit and, in particular, to a noise reduction circuit employing a plurality of channels in a band wherein the gain of a channel in controlled in accordance with the signals in certain of the other channels and wherein xe2x80x9cnoisexe2x80x9d means unwanted sounds, such as echoes, background speech or music, and random sounds.
Virtually since the invention of the telephone, techniques have been developed to improve the clarity of the sound reproduced at each station. There are a number of techniques but two are of particular interest. A first uses what is known as a comb filter; i.e. a plurality of filters wherein band pass filters alternate with band stop filters. Comb filters with complementary pass and stop bands are coupled in the two audio channels connecting the two stations of a telephone call. That is, the pass bands in one channel are the stop bands in the other channel. As a result, a signal traveling in one direction will be slightly attenuated but a signal traveling in a loop, i.e. an echo, will encounter both sets of stop bands and be highly attenuated.
The use of the complementary comb filters reduces the acoustic coupling between the speaker and microphone at each station as well as inter-station echo. Echo canceling circuits, which try to recognize a delayed signal as an echo, are much more complicated than complementary comb filter circuits and the two are often used together to eliminate echoes and other noises. However, comb filters degrade the quality of speech and do not always provide a sufficient margin of acoustic stability. One reason for the degradation is that the frequency response of a room in which the microphone and speaker of a station are located is characterized by a large number of resonant peaks. The band transitions in the comb filter transfer functions are often not sharp enough to suppress the resonant peaks, because if the transitions are too sharp the quality of the transmitted audio signal is adversely affected.
Complementary comb filter circuits are disclosed in U.S. Pat. No. 5,386,465 (Addeo et al.). This patent includes complementary comb filters in combination with other apparatus for processing audio signals to reduce noise. U.S. Pat. No. 4,991,167 (Petri et al.) discloses a slightly different system, illustrated in FIG. 4. Signals in the transmit direction are separated by filter block 80 into a set of bands, each including an attenuator, such as attenuator 82. Similarly, signals in the receive direction are separated by filter block 83 into the same set of bands, each including an attenuator, such as attenuator 84. The signals in the corresponding transmit band and receive band are compared, such as in comparator 85. The band with the smaller signal is attenuated by control circuit 86. Thus, the transmit and receive bands are paired and there is no logic interconnecting the control circuits for each pair.
Another variation on the comb filter is disclosed in U.S. Pat. No. 3,567,873 (Peroni), illustrated in FIG. 5. In this patent, the receive signal is passed through a filter bank, represented by filters 91, 92, 93, 94, and 95. The signals in each subband is compared with a threshold in level detectors 96, 97, 98, 99, and 100, respectively. Relays 101, 102, 103, 104, and 105 close their respective contacts for each band of the received signal that exceeds its threshold. In an alternative embodiment, a second set of contacts is included in the receive channel and operated oppositely from the first set from the first set. As with the Petri patent, there is no control logic looking at all the sub-bands in both channels.
A problem with these approaches is that, unlike the complementary comb filters, one can attenuate the signals in adjacent bands, thereby noticeably degrading the quality of the voice transmission. In the extreme, the telephone is reduced to xe2x80x9chalf duplexxe2x80x9d or simplex operation, i.e. single direction at a time because sounds from the other station are inaudible. The speaker at one station must stop and the circuits must re-settle before the person at the other station can be heard.
Although a complementary comb filter can attenuate an echo, current implementations of such filters cannot suppress the xe2x80x9ctailxe2x80x9d of the echo, i.e. an extended echo. The prior art can only eliminate an echo lasting a maximum of fifty milliseconds. It is desired to eliminate an echo lasting up to fifteen hundred milliseconds.
In view of the foregoing, it is therefore an object of the invention to provide a noise reducing circuit that retains the advantages of a complementary comb filter without the signal degradation.
Another object of the invention is to provide an improved technique for selectively attenuating bands in an audio communication system.
A further object of the invention is to provide a circuit that can eliminate echo tails up to fifteen hundred milliseconds in length.
Another object of the invention is to provide a circuit that can reduce both background noise and echo noise while maintaining full duplex communication.
The foregoing objects are achieved in this invention in which signals in the sending and receiving channels are each separated into a set of bands, each including a variable gain amplifier. The amplitude of a signal in a band in a first channel is controlled as a function of the amplitude of the signals in the corresponding band and the two adjacent bands in a second channel. Attenuation for echoes is restricted to alternate bands. Under certain conditions, a signal in the first channel is attenuated.